High-pressure lamps require special glasses to withstand the pressure therein. Typically such glasses are quartz glasses or other types of glasses which are high melting, that is, have a characteristic of melting only at an elevated temperature. Difficulties arise in sealing the connection leads which, typically, are of tungsten or the like, through the envelope walls of such lamps due to the difficulty in making a melt connection between the glass--of elevated melting point, and such glass coatings as can be applied to the tungsten wire.
It has previously been proposed to utilize a plurality of glasses with which a lead-through, particularly a tungsten lead-through is coated, the various glasses having different characteristics relating to temperature coefficient of expansion. It has been proposed to coat a tungsten wire with a first coating on which, in ring shape, further glass coatings are applied, forming transition glasses, which have temperature coefficients of expansion of decreasing value, the temperature coefficient decreasing as the transition glass rings are farther remote from the melt connection to the conductor. The last ring of the transition glass then has a tube of quartz glass melted thereon with one end, the other end of which is connected to a tubular portion or extension of the discharge tube itself. The result is a cap-like or dome-like melt. In another form, the tubular end of the discharge vessel is directly melted to the final rings of the transition glass itself, the glass wall of the vessel being quartz glass. Such a connection is shown and described in U.S. Pat. No. 3,140,417, with particular reference to FIGS. 1b to 1d. Such melts are suitable for lamps having an intermediate operating pressure.
It has also been proposed to make the lead-through by placing an elongated wrapping of quartz glass or of transition glasses or glass on the lead-through and then to melt on the tubular end of the discharge vessel, which is made of quartz-glass, over the entire length of this wrapping. Such melt connections utilize an excessive amount of materials and glass, are difficult to carry out in mass production and even under laboratory conditions, and, after melting-in, have undesirable stresses at the melt-in location. Such an arrangement is described, for example, in German Pat. No. 886,043.
Quartz/metal melts have to accomodate vastly different thermal coefficients of expansion. Directly melting-on of a glass vessel which is made of quartz glass on a transition glass is difficult in view of the great difference of the melting temperatures between the materials utilized--the difference in melting temperature may be in the order of 1300.degree. C.
The linear thermal coefficient of expansion of tungsten is in the order of 45.10.sup.-7 /.degree. C., within the temperature range of from zero to 300.degree. C.; quartz glass, which is the typical glass for use in high-pressure discharge lamps, has a thermal coefficient of expansion of about 5-6.10.sup.-7 /.degree. C. The transition glasses to melt in tungsten wires thus are usually so selected that there is a step-wise transition of the thermal coefficient of expansion between the material of the discharge vessel and of the metallic conductor. Glasses which can be melted on a tungsten wire, that is, transition glasses, have working temperatures in the order of between 660.degree. to 800.degree. C.; quartz glass has a working temperature of about 2000.degree. C. These vast differences in thermal coefficients of expansion, and working temperatures to which the glasses can be subjected during the working processes make attachment of such glasses and tungsten lead-in wires extremely difficult.